The present invention relates to a mobile radio telephone communication system having a plurality of speech channels and at least one common control channel, and more particularly to a method of detecting troubles in a receiver for the common control channel thereof.
Currently available mobile systems of radio telephone communication include the so-called cellular system, wherein the service area is divided into a plurality of small zones, a base station being installed in each zone, and communication is achieved between a mobile subscriber station and a fixed subscriber or between mobile subscriber stations over one of plural speech channels assigned to the base station concerned. In this system, there are assigned to each base station for common use by the plurality of mobile stations in the zone covered by the base station, in addition to a plurality of speech channels, a paging channel for controlling incoming calls to the mobile stations and an access channel for controlling outgoing calls therefrom. One example of mobile radio telephone communication system having such a channel arrangement is described in "800 MHz Band Land Mobile Telephone Control System" in "REVIEW OF THE ELECTRICAL COMMUNICATION LABORATORIES", pp. 1172-1190, Vol. 25, Nos. 11-12, published by Nippon Telegraph & Telephone Public Corporation, Nov.-Dec. 1977.
In a mobile radio telephone system for achieving communication over multiple speech channels, it is intended to reduce the time required for making connections and to efficiently use the radio frequencies assigned thereto by the specialized connection control of the paging and access channels. However, because the paging and access channels are commonly used in a given radio zone, in the event of a trouble in the radio transmitter/receivers or on the wire line for the paging and access channels, the mobile stations present in the zone covered by the troubled base station will become unable either to transmit or to receive calls, affecting the functioning of the whole system.
Therefore, any trouble on the wire line or in the radio transmitter/receivers for the paging and access channels has to be quickly detected and made known to the central station controlling the base station. For detecting troubles on the paging and access channel of a base station, it is known to constantly monitor the outputs of the transmitter/receivers, by which trouble detection on paging channel alone can be achieved with comparative ease. This is because on the paging channel a control signal is transmitted from the base station. This system, however, cannot easily detect the trouble on the access channel of the base station. This results from the random transmission of call signals on the access channel from the mobile station. Thus, when there is no output from the access channel receiver, it is impossible to dtermine whether the absence of an output is attributed to a problem on the access channel receiver or to the absence of a call from the mobile station.
There has been developed a system in which a test mobile station is installed in each base station. An operator, by remote control from the central station, actuates each test mobile station to send a test transmission signal in response to a report of trouble from a mobile station subscriber or in accordance with a periodic maintenance program to detect any trouble on the access channel. For further details on such a system, reference is made to "Supervisory and Control Equipment", in "NEC RESEARCH & DEVELOPMENT", No. 57, pp. 99-108, issued by Nippon Electric Co., Ltd., April 1980. Although it is a useful system capable of detecting troubles on the access channel of the base station, it has a disadvantage in that if the intervals of maintenance by the operator are too long, a problem may take a long time to be detected by the operator after it arises or, if maintenance intervals are reduced to quicken trouble detection, the detecting process will hinder communication by other mobile stations sending out ordinary call signals.